The present invention relates to a vertical deflection circuit for a CRT display, and more specifically to a linearity compensation circuit of the negative feedback type which is little affected by the change in the capacity of the electrolytic capacitor that is subject to change depending upon the change in the temperature.
In the conventional vertical deflection circuit, the problem fundamentally exists in that the third order linearity distortion (so-called S-shaped distortion) is compensated by using a chemical capacitor with which it is difficult to maintain stability relative to the change in the temperature.
Problems inherent in the prior art will now be described with reference to FIG. 1, wherein reference numeral 1 denotes a vertical sawtooth wave generating circuit, 2 denotes an amplifier circuit, 3 denotes a vertical deflection yoke (having an inductance L.sub.1), 4 denotes loss resistance (having a resistance value R.sub.1) of the yoke, 5 denotes a chemical capacitor (having a capacity C.sub.1) for third order distortion compensation and for DC blocking, reference numeral 7 denotes a negative feedback circuit for stabilizing output voltage bias and for compensating third order distortion and wherein symbols R.sub.3, R.sub.4 and C.sub.2 denote constituent elements therefor, 8 denotes an adder, 9 denotes a potentiometer for adjusting the vertical size, and 10 denotes a potentiometer for compensating second order distortion.
Operation of the circuit as shown in FIG. 1 will now be described. A vertical drive signal VD input to the vertical sawtooth wave generating circuit 1 of FIG. 1 serves as a vertical synchronizing signal for the circuit 1. In synchronism with the above synchronizing signal, a sawtooth wave of a vertical period shown in FIG. 2a is formed on the output 11 of the sawtooth wave generating circuit.
The sawtooth wave is amplified through an amplifier 2 and is applied to the deflection yoke 3. In response thereto, an electric current flows into the deflection yoke 3 and is detected by a detector resistance 6.
In the constructed circuit, there exist three different feedback paths. That is, a first feedback path is a negative feedback path that leads to a negative input terminal of the amplifier 2 via the potentiometer 9 and the adder 8.
A second path is a sub-negative feedback circuit that passes through the negative feedback circuit 7 and the adder 8.
A third path is a positive feedback circuit that leads to the sawtooth wave generating circuit 1 via the potentiometer 10 and a resistor 12 (having a resistance R.sub.5). As will be described later, the positive feedback circuit works to cancel the second order distortion that generates accompanying the third order distortion compensation function of the second sub-negative feedback circuit.
An electronic switch SW in the sawtooth wave generating circuit is closed for a short period of time in synchronism with the input vertical synchronizing signal such that the potential of the capacitor Cs is instantaneously charged to the power source voltage Ecc. During the rest period, i.e., during the period in which the electronic switch SW is opened, the potential of the capacitor Cs is gradually discharged due to the collector current of a transistor Q. Thus, the sawtooth wave shown in FIG. 2a is obtained across the terminals of the capacitor Cs, i.e., on the input terminal 11 of the amplifier 2.
The gain of the amplifier 2 is as great as about 60 dB or more. Therefore, the loop gain of the system is sufficiently greater than 1 at a vertical frequency (60 Hz) and at principal higher harmonic frequencies. Therefore, the feedback loop so works that a waveform nearly equal to that of the terminal 11 is formed on the negative input terminal 13 of the amplifier 2. The relationship between a voltage E.sub.13 at the terminal 13 and a current I.sub.DY that flows into the current detector resistance 6 via deflection yoke 3 is given by the following equation based upon the calculation of flow of signals, i.e., ##EQU1##
In practice, the value T.sub.2 can be selected to be as great as about 100 msec. Therefore, the equation (1) can be modified and approximated in the following way. That is, it (1+PT.sub.2) is approximated to be PT.sub.2, then, ##EQU2## where E.sub.2 =I.sub.DY .times.R.sub.2
To explain the meaning of the equation (2), first, the practical circuit examples are explained below (in the following examples, pp stands for peak to peak).
______________________________________ L.sub.1 = 8 mH R.sub.3 = 30 k.OMEGA. R.sub.1 = 8 .OMEGA. R.sub.4 = 10 k.OMEGA. C.sub.1 = 0.36 mF C.sub.2 = 13 .mu.F R.sub.2 = 2 .OMEGA. K.sub.4 = 0.25 K.sub.9 = 1.0 pp value of I.sub.DY = 2App ______________________________________
If these values are substituted for the equation (2), there are obtained the following equation, ##EQU3##
Based upon the action of negative feedback described above, it can be regarded that the voltage E.sub.11 at the terminal 11 is equal to the voltage E.sub.13. Therefore, the following equation is obtained from the equation (4), ##EQU4##
It can be interpreted that the equation (5) represents the response of output E.sub.2 (=I.sub.DY R.sub.2) that corresponds to the sawtooth input E.sub.11 of vertical periodicity to the amplifier 2.
The numeral 1 which is the first term in the parenthesis on the right side of the equation (5) just represents the sawtooth wave.
The third term (i.e., the term of double integral :1/p.sup.2) represents a so-called S-shaped distortion compensation term.
Described below is the reason why S-shaped distortion is generated.
The reason why S-shaped distortion is generated is because the fluorescent screen of the CRT is flat. If the fluorescent screen were on a spherical surface whose center coincides with the center of the deflection yoke, then S-shaped distortion is not generated. This is because the sine of deflection angle of the CRT varies in proportion to a current that flows into the deflection yoke and the deflection distance on the spherical surface also varies in proportion to the sine of deflection angle. If the deflection angle in the vertical direction is denoted by .theta., then the deflection distance on the practical flat fluorescent screen varies in proportion to tan.theta.. Therefore, the linearity of vertical deflection ##EQU5##
The equation (6) represents a so-called S-shaped distortion in which the top part and bottom part of the screen are extended relative to the central part (.theta.=0). Concretely speaking, when the vertical deflection angle is .theta.=.+-.15.degree., there develops enlarging distortion of about 11% on the top and bottom part of the screen. ##EQU6##
Thus, the reason why S-shaped dstortion generates is quantitatively comprehended. To compensate the S-shaped distortion, therefore, it is obvious that the deflection speed at the top part and bottom part of the screen must be decreased by about 11% compared with that at the central part of the screen.
Described below is a principle for cancelling the S-shaped distortion by the third term on the right side of the equation (5).
For the purpose of easy explanation, attention is given to the time differentials E.sub.11 and E.sub.2 instead of E.sub.11 and E.sub.2 in the equation (5).
First, let it be pressumed that E.sub.11 represents a perfect sawtooth wave. Then, E.sub.11 represents a perfect pulse wave of a vertical periodicity. In FIG. 2, E.sub.11 and E.sub.11 represent the above waveforms, and wherein the abscissa represents the time t.
FIG. 2 further illustrates contributions by the second and third terms on the right side of the equation (5). The added result of the first and third terms is illustrated in the bottom column in FIG. 2. In the following consideration, it should be noted that the linearity characteristics is obtained by differentiating the equation (5) with the time t. That is, ##EQU7##
The left side of the above equation represents the differentiation of a voltage across the resistor R.sub.2 of FIG. 1. A deflection current is flowing into the resistor R.sub.2. Therefore, E.sub.2 varies in proportion to the rate of change in the deflection current.
The rate of change is given by the sum of the terms in the right side of the equation (5').
The terms of the equation (5') are shown in FIGS. 2a to 2e. It will be understood from FIG. 2 that the S-shaped distortion is compensated by about 11%, i.e., the deflection speed is lowered at the top and bottom parts of the screen. In the foregoing was described the principle for compensating the S-shaped distortion according to the prior art.
The side effects will now be described.
There are two side effects. The first one is a second order distortion represented by the second term in the right side of the equation (5). As will be obvious from the waveform of FIG. 2c, this term contributes to extend the top part of the screen by about 10% and to contract the bottom part of the screen by about 10%. This is because, the waveform shown in FIG. 2c is reduced by 10% at the top part of the screen (at the beginning of scanning) in contrast with the rate of reference change of -100% of E.sub.11 shown in FIG. 2a. Therefore, the total rate of change of -110% is achieved.
Generally, the linearity distortion of greater than 5% is detrimental to faithfully reproducing the figures.
In order to cancel the second order linearity distortion, there is added the positive feedback loop that starts from the potentiometer 10 of FIG. 1 via the transistor Q. This loop works to contract the top part of the screen and to expand the bottom part of the screen.
The second side effect is that the circuit is easily affected by the change in the capacitance of the chemical capacitor. As the consideration of the third term of the equation (5) based upon the equations (2) and (1) will indicate, the double integral effect is obtained by using two chemical capacitors C.sub.1 and C.sub.2. At low temperatures, the capacitances of these capacitors decrease to about 0.7 times. Accordingly, the compensation effect increases to two folds. Therefore, even if the distortion is completely compensated at ordinary temperature, the compensation becomes excessive at low temperatures and the top and bottom parts of the screen are contracted by about 11%. Furthermore, since the cancellation effect is not sufficiently exhibited by the positive feedback loop, the top part of the screen extends more than the bottom part.
As is widely known, furthermore, the capacity of the chemical capacitor tends to decrease after the use of long periods of time. When used for extended periods of time, therefore, the linearity distortion inevitably develops.
At high temperatures, on the other hand, the internal series resistance R.sub.1 of the winding of the deflection yoke increases and the second term in the right side of the equation (2) increases correspondingly, causing the second order distortion to increase.
In general, the resistance of the winding increases to about 1.2 times due to initial drift (temperature rise) after the power supply switch is closed. Therefore, the second order distortion of the screen increases by about 1.2 times. Accordingly, the second order distortion on the screen increases from the aforementioned initial value of about 20% to about 24% which is 1.2 times as great.
In the foregoing was described the case where the vertical deflection angle was as relatively small as about .+-.15 degrees. As will be understood from the equation (6), the third order linearity distortion usually varies in proportion to the square power of the deflection angle. In the systems having large deflection angles, therefore, the aforementioned problems are further amplified.
In recent years, it has been urged to provide displays that can cope not only with a single vertical scanning frequency but also with a plurality of frequencies (e.g., 40 Hz to 120 Hz). According to the above-mentioned conventional vertical deflection circuit, the principle for compensating the third order distortion is based upon a double integration circuit. Therefore, if the optimization is set at 60 Hz, then the compensation effect decreases to one-fourth at 120 Hz.
Japanese Patent Laid-Open No. 127877/1985 can be quoted as a related prior art. According to the above prior art, however, both the DC components and the AC components exist on a negative feedback loop in the vertical deflection circuit and, hence, effort is made to improve vertical linearity compensation, vertical amplitude adjustment, and to stabilize the bias point of the class B push-pull amplifier.
According to the prior art as will be comprehended from the foregoing description, the double integration circuit is used as means for compensating S-shaped distortion. A circuit for compensating the second order distortion based upon the positive feedback system is also required. According to the prior art, therefore, linearity distortion develops on the screen due to change in the capacity of the chemical capacitors with the change in temperature or with the lapse of time and due to change in the resistance of winding of the deflection yoke with the change in temperature.